Rewrite std::bitv using classes

[catamorphism]

A very old -- but still accurate -- FIXME in std::bitv suggests rewriting the module to use recursive object types -- aka "classes". Now that we have classes, it's possible to do this.

I'd enjoy working on this, but if someone else wants to try out classes, that would be cool too.

[catamorphism]

I've implemented this, but am still debugging it

[Dretch]

I've implemented this, but am still debugging it

Are you by any chance getting Invalid read/write errors during the test suite?

I have added some code to std::bitv for #2964 and I am now getting such errors, and also some assertion failures during the tests I have added. Curiously enough the code/tests I have added seem to work correctly outside of the std::bitv module - so I think something weird is happening here.

[catamorphism]

@Dretch No -- I'm getting wrong results. It's not too surprising that you could get different behavior inside and outside the bitv module, as the compiler does some things differently for inside-the-crate vs. outside-the-crate things (though if it changes observable behavior, that's a bug).